1. Field of the Invention
The present invention relates to a copolymer suitable for photoresist. More particularly, the present invention relates to a vinyl 4-hydroxybenzal-vinyl alcohol-vinyl acetate copolymer, a vinyl 4-t-butoxycarbonyloxybenzal-vinyl alcohol-vinyl acetate copolymer and a vinyl 4-t-butoxycarbonyloxybenzal-vinyl 4-hydroxybenzal-vinyl alcohol-vinyl acetate copolymer. In the last two copolymers, all or part of the vinyl 4-hydroxybenzal groups are protected with t-butoxycarbonyl group. Also, the present invention is concerned with methods for preparing the copolymers.
2. Description of the Prior Art
A significant advance has been made for the photoresist used in microlithography, which is one of the most essential techniques to achieve the high integration of semiconductor integrated circuits. As higher integration degree of a semiconductor integrated circuit has been required, various photoresist materials are now developed and reported. Among them, polyvinylphenol resins protected by t-butoxycarbonyl group and poly(4-t-butoxycarbonyloxystyrene) resins are expected to be the most useful and are being used in practice. These resins are very advantageous in that they are directly applied for such techniques advanced in microlithography as utilizes deep uv as a light source for exposure, as disclosed in U.S. Pat. Nos. 4,491,628, 4,405,708 and 4,670,507.
However, the polyvinylphenol resins show high optical absorbance at around 250 nm due to the benzene rings contained. Because deep uv, a light source for the microlithography in current use, has a frequency of around 250 nm, the polyvinylphenol resins are low in transparency at the frequency.
Poly(4-t-butoxycarbonyloxystyrene) resins lose too much weight upon thermal treatment after exposure in addition to being poor in adhesiveness. Like polyvinylphenol resins, they show low transparency because of high optical absorbance at around 250 nm. For example, a resin obtained by using 4-t-butoxycarbonyloxy styrene with an average molecular weight of 10,000 as a monomer and 2,2xe2x80x2-azobisisobutyronitrile (AIBN) as an initiator, shows a uv optical absorbance of 0.170 at 250 nm when it is 1 xcexcm thick. For a resin 1 xcexcm thick polymerized from the monomer in the presence of benzoylperoxide, its uv optical absorbance is 0.140. As exemplified above, both resins are opaque at around 250 nm because of high optical absorbance.
As a result of intensive and thorough research by the inventors, the present invention is based on the finding that a vinyl 4-t-butoxycarbonyloxybenzal-vinyl alcohol-vinyl acetate copolymer and a vinyl 4-t-butoxycarbonyloxybenzal-vinyl 4-hydroxybenzal-vinyl alcohol-vinyl acetate copolymer both show low absorbance at 250 nm in addition to being superior in thermal stability and mechanical strength. Also, the copolymers can be applied for photoresist. Various tests showed that the photoresist made mainly of the copolymers was transparent at 250 nm, lost little weight upon the thermal treatment after exposure and adhered well to the silicon wafer.
To prepare a novel vinyl 4-t-butoxycarbonyloxybenzal-vinyl alcohol-vinyl acetate copolymer, poly(vinyl alcohol) with an average molecular weight of about 8,000-200,000 is dissolved in dimethylformamide to give a 5-20 wt % solution. This solution is then added to an acid and stirred at 50-140xc2x0 C. for 1-5 hrs. 4-hydroxybenzaldehyde is added, followed by stirring under the same conditions as above. Precipitation occurs in distilled water. The precipitate is dissolved in dimethylformamide again and re-precipitated in distilled water. The procedure of dissolving in dimethylformamide and precipitating in distilled water is repeated two or three times. Then, the final precipitates are dried in vacuo to obtain pure vinyl-4-hydroxybenzal-vinyl alcohol-vinyl acetate copolymer. The acid, hydrochloric acid, sulfuric acid, or phosphoric acid is used at an amount of 1-20 weight parts based on 100 weight parts of the total reaction. Instead of dimethylformamide, distilled water may be employed. In this case, as the polymerizing reaction progresses, the copolymer is produced as precipitates. A desired product can be obtained by filtering and washing the precipitates.
The vinyl 4-hydroxybenzal-vinyl alcohol-vinyl acetate copolymer thus obtained is dissolved in dimethylformamide to give a 3-20 wt % solution which is, then, stirred for about 1 hr in the presence of sodium hydride (NaH), t-butoxide, triethylamine or calcium carbonate (CaCO3). Subsequently, di(t-butyldicarbonate) is added, followed by stirring at room temperature for 0.5-2 hr. A product is obtained in distilled water as precipitates. Then, they are dissolved in dimethylformamide again and re-precipitated in distilled water. The procedure of dissolving in dimethylformamide and precipitating in distilled water is repeated two or three times. Then, the final precipitates are dried in vacuo to produce pure vinyl 4-t-butoxycarbonyloxybenzal-vinyl alcohol-vinyl acetate copolymer or vinyl 4-t-butoxycarbonyloxybenzal-vinyl 4-hydroxybenzal-vinyl alcohol-vinyl acetate copolymer. When di(t-butylcarbonate) is added at an amount more than the mole numbers of vinyl 4-hydroxybenzal group, bulky vinyl 4-hydroxybenzal groups are all protected with t-butoxycarbonyl group producing vinyl-4-butoxycarbonyloxybenzal-vinyl alcohol-vinyl acetate copolymer. In contrast, addition of di(t-butylcarbonate) at an amount less than the mole numbers of vinyl 4-hydroxybenzal group protects a portion of vinyl-hydroxybenzal groups with t-butoxycarbonyl group producing vinyl 4-t-butoxycarbonyloxybenzal-vinyl 4-hydroxybenzal-vinyl alcohol-vinyl acetate copolymer.
Depending on the ratio of the functional groups contained, the copolymers have characteristic properties. Hence, the functional groups must be present at a certain amount range in order for the copolymers to be used as photoresist. In accordance with the present invention, 4-t-butoxycarbonyloxybenzal group is preferably present at an amount of 5-70 mole % in the vinyl 4-t-butoxycarbonyloxybenzal-vinyl alcohol-vinyl acetate copolymer. For example, if 4-t-butoxycarbonyloxybenzal group is present at an amount less than 5 mole %, the vinyl 4-t-butoxycarbonyloxybenzal-vinyl alcohol-vinyl acetate copolymer cannot be used practically because deficient protection cannot effect the difference in solubility in a developing solution. On the other hand, if 4-t-butoxycarbonylbenzal group is present at an amount more than 70 moles, the copolymer is poor in adhesion to the substrate. More than 60 mole % of hydroxy group makes an unexposed area to be solubilized in a developing solution. If acetate group is present at an amount more than 40 mole %, the extrication transition temperature of the copolymer significantly decreases. Accordingly, it is preferrable that hydroxy group is present at an amount of 1-60 mole % and 4-t-butoxycarbonyloxybenzal group is present at an amount of 5-70 mole %, in the vinyl 4-t-butoxycarbonyloxybenzal-vinyl alcohol-vinyl acetate copolymer, according to the present invention. Thus, acetate group is preferably present at an amount of 0-40 mole % in the copolymer.
In accordance with an aspect of the present invention, a vinyl 4-hydroxybenzal-vinyl alcohol-vinyl acetate, represented by the following general formula I is provided: 
wherein
a is a mole percent ranging from 5 to 70;
b is a mole percent ranging from 1 to 60; and
c is a mole percent ranging from 0 to 40.
In accordance with another aspect of the present invention, a vinyl 4-t-butoxycarbonyloxybenzal-vinyl alcohol-vinyl acetate copolymer, represented by the following general formula II is provided: 
wherein
x is a mole percent ranging from 5 to 70;
y is a mole percent ranging from 1 to 60; and
z is a mole percent ranging from 0 to 40.
The copolymer chains of formulas I and II each shows regularity or irregularity.
In the vinyl 4-t-butoxycarbonyloxybenzal-vinyl 4-hydroxybenzal-vinyl alcohol-vinyl acetate copolymer according to the present invention, the vinyl 4-hydroxybenzal groups of vinyl 4-hydroxybenzal-vinyl alcohol-vinyl acetate copolymer remain partly unprotected by t-butoxycarbonyl group. Such unprotected acetal structure brings about the effects of enhancing mechanical strength, increasing extrication transition temperature, and intensifying sensitivity by virtue of the remaining phenol. Because an appropriate solubility in developing solution comes from the deprotection of the 4-t-butoxycarbonyl group, the 4-hydroxybenzal group unprotected by t-butoxycarbonyl group does not exceed 50 mole %. Therefore, in accordance with a further aspect of the present invention, a vinyl 4-t-butoxycarbonyloxybenzal-vinyl 4-hydroxybenzal-vinyl alcohol-vinyl acetate copolymer, represented by the following general formula III is provided: 
wherein
m is a mole percent ranging from 1 to 50;
n is a mole percent ranging from 5 to 60;
o is a mole percent ranging from 1 to 60; and
p is a mole percent ranging from 0 to 40.
The polymers of Formulas II and III are superior in thermal stability and mechanical strength by virtue of the presence of acetal structure in their main chains. In addition, because the t-butoxycarbonyl group, which, upon thermal treatment after exposure, is deprotected to change the solubility of the copolymer in a particular solvent, is substituted only in the acetal structure. Also, the copolymer shows little weight loss in the thermal treatment. Therefore, the photoresist film made of the copolymer does not lose its thickness when being developed. Optical absorbance at 250 nm was found to be about 0.021 for the resin 1 xcexcm thick prepared from the copolymers with an average molecular weight of about 10,000, as determined by infrared spectra. This value is no more than xe2x85x9-{fraction (1/7)} of the optical absorbance of poly(4-t-butoxycarbonyloxystyrene) resin. Thus, the copolymers are far superior in transparency. In addition, the hydrophilicity effected by the alcohol and acetate groups present in the copolymer chain allows the copolymer to better adhere to silicon wafer.
A better understanding of the present invention may be obtained in light of following examples which are set forth to illustrate, but are not to be construed to limit, the present invention.